1. Field of the Invention
This invention relates to a zoom lens, and in particular to a zoom lens adopting the so-called inner focus system or rear focus system by the movement of some of lens groups in the zoom lens system.
2. Related Background Art
As focusing systems for conventional zoom lenses, there are known the so-called forward lens axial movement system in which a first lens group positioned most adjacent to the object side is axially forwardly moved, the inner focus system or the rear focus system by the movement of some of lens groups in the zoom lens system except a first lens group most adjacent to the object side, the total axial movement system in which the total system is axially forwardly moved as a unit, etc.
Generally, the inner focus system, the rear focus system and the total axial movement system suffer from the problem that the amount of axial movement necessary for one and the same photographing distance (the distance from an object to the image plane) differs with a variation in the focal length of the total system. Therefore, even if with any focal length, the lens is in focus to a certain photographing distance, when a magnification change is effected to change the focal length of the total system, the imaging position fluctuates greatly and each time, it has been necessary to manually operate a focusing ring to thereby effect focusing over again. Accordingly, the above-mentioned three systems are all unsuitable for manual focusing.
Thus, the forward lens axial movement system in which the amount of lens movement is always constant for one and the same photographing distance in any photographing magnification condition and which is convenient for manual operation (manual focusing) is the main current as the focusing system for zoom lenses.
Generally, however, the first lens group has the tendency of becoming heavy due to its large lens diameter. For example, in a zoom lens of multi-group construction and high magnification change ratio comprising three positive, negative and positive lens groups or more, aperture eclipse is liable to occur on the wide angle side during short distance focusing, and an attempt to mitigate this phenomenon would result in the bulkiness of the first lens group, and would further lead to the problem that during short distance focusing, curvature of image field occurs very greatly in the positive direction to cause the deterioration of the imaging performance.
Also, in recent years, the main current of camera has become single-lens reflex cameras of the auto focus type, and if auto focusing is effected by the forward lens axial movement system, a great load will be applied to a drive motor for focusing because the first lens group is bulky and heavy. This in turn leads to the problem that the focusing speed becomes slow and quick focusing becomes impossible.